Read vrm portal id from /data/venus/unique-id.
Read vrm portal id from /data/venus/unique-id.

If that file does not exist, fall back on legacy behaviour of
obtaining the ethernet address of eth0. Allow this to be overridden
using the environment. This makes it possible to run on a desktop
during development, or on newer Raspbians where the network interface
might be named differently.

Ref victronenergy/venus#254

victronenergy/velib_python